1. Field of the Invention
The present invention relates to a power transmission mechanism having at least one cam-incorporated gear, and in particular, relates to a type of power transmission mechanism having at least one cam-incorporated gear and a planetary gear mechanism used to transmit rotation to the cam-incorporated gear. The present invention also relates to a power transmission mechanism of an imaging apparatus.
2. Description of the Related Art
In addition to a conventional object viewing function (optical view mode) using an optical viewfinder, SLR digital cameras having a ‘live view mode’, in which an electronic object image captured by an image sensor is viewed in real-time on a display monitor such as an LCD before photographing, are known in the art. In the live view mode, the quick-return mirror (hereinafter referred simply to as the mirror) is held in the retracted position (mirror-up position) in which the quick-return mirror is retracted from a photographing optical path when the object is viewed before photographing, which is different from a viewing state in the optical view mode, in which the object is viewed through an optical viewfinder before photographing. Therefore, when a photographing operation is performed in the live view mode, the mirror drive operation and the shutter drive operation need to be controlled in a different manner from those in optical view mode (normal exposure mode).
In the SLR digital camera disclosed in Non-Patent Document 1 (pages 202 and 203 in “Asahi Camera” (a Japanese monthly photographic magazine published by Asahi Shimbun Publications Inc., print edition, August 2008), another image sensor (live-view image sensor) used exclusively for capturing the object image in the live view mode, independently of an image sensor used for taking pictures, is installed in a viewfinder optical system so that a quasi-live view operation can be performed using the live-view image sensor in the viewfinder optical system. In this type of camera, an electronic image captured by an image sensor different from the image sensor used at a time of exposure is viewed on a display monitor before shooting, and accordingly, this view mode cannot be considered to be live view mode in the strict sense, as there is a possibility of the image displayed on the monitor not precisely coinciding with the image taken at a time of exposure. Additionally, installation space for an image sensor needs to be secured in the viewfinder, which becomes an obstacle for miniaturization of the camera and causes an increase in production cost.
In the SLR digital camera disclosed in Non-Patent Document 2 (pages 230 and 231 in “Asahi Camera” (a Japanese monthly photographic magazine published by Asahi Shimbun Publications Inc., print edition, January 2008), the mirror drive mechanism and the shutter drive mechanism are provided with two independent drive sources, respectively, and a shutter charge operation is performed by the shutter drive mechanism with a mirror-up state being maintained by the mirror drive mechanism in a photographing operation in the live view mode. In this type of camera, a mirror drive system and a shutter drive system are each provided with an independent drive source and an independent drive mechanism, which makes it difficult to achieve miniaturization and weight reduction of the camera and causes an increase in production cost.
The applicant of the present invention has found that miniaturization, weight reduction and cost reduction of the camera body can be achieved in an SLR camera having a live view mode by solving the above described problems that reside in Non-Patent Documents 1 and 2 by employing a planetary gear mechanism as a mechanism for transmitting driving force of a motor to a mirror drive system (mechanism) and a shutter charge system (mechanism). Specifically, the intention is to carry out a photographing operation (sequence of processes for taking a picture) in the live view mode by a drive mechanism equipped with two cam-incorporated gears like the drive mechanism shown in Japanese Patent No. 3,153,482 and to transmit a driving force selectively to the two cam-incorporated gears via a planetary gear mechanism. However, it is difficult to use a planetary gear mechanism as a device for transmitting a driving force selectively to such two cam-incorporated gears in the following respects.
A planetary gear mechanism has a basic structure such that rotation of a drive source (motor) is transmitted from a sun gear rotated by the drive source to a planet gear and that a state of the planet gear is switched between two different states: a state of being engaged with a driven gear so that rotation of the planet gear is transmitted to the driven gear and a state of being disengaged from the driven gear (to be engaged with another driven gear), in accordance with the direction of rotation of the sun gear. Although a certain degree of load (rotational resistance) acts on the driven gear from a driven member to which a driving force is transmitted via driven gear, there is a possibility of a torque in the rotational direction of the driven gear (surplus torque) acting on the driven gear if the load characteristics of the driven member fluctuate. If such a surplus torque acts on the driven gear, a state where the relationship between the planet gear and the driven gear, that serve as a driver and a follower, respectively, is reversed so that the driven gear serves as if it were a driving source occurs. Under such conditions, if it is attempted to make the planet gear engaged with the driven gear, there is a possibility of the planet gear behaving in a manner so as to bounce on the driven gear, which may disable the control of the driven member by the driven gear.
For instance, in the SLR camera disclosed in Japanese Patent No. 3,153,482, a mirror up/down operation and a shutter charge operation are performed by one of the two cam-incorporated gears, and an adjustable diaphragm is controlled by rotation of the other cam-incorporated gear. The driven members of the mirror drive system, the shutter charge system and the diaphragm control system are each provided with a swingable lever which swings by being guided by a surface cam formed on the associated cam-incorporated gear, and each swingable lever is biased to rotate in a direction to come in contact with this surface cam. When each swingable lever is pressed to rotate in a direction against such a biasing force by the associated surface cam, a torque (load torque) in a direction opposite to the rotational direction of the cam-incorporated gear acts on the cam-incorporated gear; however, when the swingable lever rotates in the opposite direction, the biasing force acting on the swingable lever acts on the cam-incorporated gear as surplus torque. Since the swingable levers of the mirror drive system, the shutter charge system and the diaphragm drive system are each rotated to and fro during one turn of the associated cam-incorporated gear, it is sometimes the case that the surplus torque caused by any one swingable lever strongly acts on the associated cam-incorporated gear depending on the rotation position of the cam-incorporated gear. Due to this reason, assuming that a planetary gear mechanism is used as a driving force transmitter for transmitting a driving force to the cam-incorporated gear, there is a possibility of a poor engagement of the planet gear with the cam-incorporated gear occurring.